The present invention relates generally to antennas for radar and radio communications which may be positioned in angle, and more particularly to radar and radio communications antenna systems with large fixed apertures, which have integral means for rapidly scanning the beam formed by the large fixed aperture.
Modern radar and communications systems often require antenna beams which can be rapidly positioned in angle (i.e. scanned) in order to provide surveillance and tracking of multiple radar targets or to sequentially contact multiple communications terminals. Greater scanning speed is desirable since it permits an increase in the target handling capacity of a radar system or of the number of communications terminals which can be accessed in a given time period. Existing methods for scanning a radar or radio beam usually have some undesirable limitation with regard to scanning speed.
In most antenna systems the beams are scanned by moving the entire structure (viz. feed and reflector) as a unit. The main problem with such antenna systems is that scanning speed is severely limited simply by the inertia of the structure. In such antenna systems scanning speed can only be increased by either a reduction in size or an increase in locomotive power. The former, however, can result in undesirable performance limitations (e.g. reduction in range), while the latter can result in an incremental increase in cost which is disproportionate to any benefit gained.
Other mechanical approaches to increasing scanning speed include moving the feed in the focal plane of the antenna aperture and moving only the reflector. Moving only the feed mandates that the feed be translated in the focal plane of the aperture. Furthermore, scanning speed is still limited by the inertia of the feed structure.
Moving a reflector large enough to scan the beam saves little over moving the entire antenna mount. U.S. Pat. No. 3,771,160, Laverick, and U.S. Pat. No. 3,562,753, Tanaka et al. disclose antennas wherein scanning is accomplished by moving a large reflector. However, increased scanning speed is not an object of either of those patents.
Scanning speed could be increased by deploying a plurality of feeds across the focal plane of the antenna aperture. Such a configuration is shown in U.S. Pat. No. 3,688,311, Salmon (claim 5 and FIG. 2). Scanning would be accomplished by switching among the many feeds. However, such a configuration would be very expensive due to the additional electronic equipment necessitated by such an arrangement.
Other electronic approaches to large aperture beam scanning have been developed. Among these are array-type antennas such as phased-arrays and frequency-scanned-arrays. In a phased-array the beam scan angle is controlled by phase shifters. While these phased-arrays are fast, they are also very expensive.
In a frequency-scanned-array the beam scan angle is controlled by changing the frequency about a center frequency. Frequency scanning, however, is complicated by the large frequency spectrum necessary to scan a reasonable angular sector. Thus frequency scanning is bandwidth limited. Furthermore, even if wide bandwidth were available, the use of frequencies for other purposes, such as electronic counter-countermeasures or accurate range measurement and resolution, would be precluded. See, Skolnik, Introduction to Radar Systems 312 (McGraw-Hill 1962).
U.S. Pat. No. 4,203,105, Dragone et al. utilizes a scanning array feed in an offset reflector antenna arrangement. The effect of such a configuration is to magnify the image of the beam emitted by the array feed. There are, however, certain limitations even to this arrangement.
First of all, feed arrays are very expensive compared to single feeds. Second, Dragone et al. utilizes paraboloidal reflectors. It is well known that paraboloids with small f/D ratios have narrow image planes. Thus, for the arrangement shown in FIG. 1 of Dragone et al., beam spreading at 10 beamwidths off the axis would reduce gain by 25-30%.